Understanding the Impacts of Soil Moisture Initial Conditions on NWP in the Context of Land–Atmosphere Coupling

Abstract

The role of soil moisture in NWP has gained more attention in recent years, as studies have demonstrated impacts of land surface states on ambient weather from diurnal to seasonal scales. However, soil moisture initialization approaches in coupled models remain quite diverse in terms of their complexity and observational roots, while assessment using bulk forecast statistics can be simplistic and misleading. In this study, a suite of soil moisture initialization approaches is used to generate short-term coupled forecasts over the U.S. Southern Great Plains using NASA’s Land Information System (LIS) and NASA Unified WRF (NU-WRF) modeling systems. This includes a wide range of currently used initialization approaches, including soil moisture derived from “off the shelf” products such as atmospheric models and land data assimilation systems, high-resolution land surface model spinups, and satellite-based soil moisture products from SMAP. Results indicate that the spread across initialization approaches can be quite large in terms of soil moisture conditions and spatial resolution, and that SMAP performs well in terms of heterogeneity and temporal dynamics when compared against high-resolution land surface model and in situ soil moisture estimates. Case studies are analyzed using the local land–atmosphere coupling (LoCo) framework that relies on integrated assessment of soil moisture, surface flux, boundary layer, and ambient weather, with results highlighting the critical role of inherent model background biases. In addition, simultaneous assessment of land versus atmospheric initial conditions in an integrated, process-level fashion can help address the question of whether improvements in traditional NWP verification statistics are achieved for the right reasons.

Relative contribution of soil moisture, boundary‐layer and microphysical perturbations on convective predictability in different weather regimes

“…; Santanello et al . ). Even nowadays, few soil moisture insitu observations, the large subgrid variability and imperfect data assimilation techniques hamper a realistic convective‐scale soil moisture analysis (Gustafsson et al .…”

“…Since many convective‐scale phenomena are forced from the lower boundary condition, surface and soil properties represent an important source of uncertainty which has been mostly overlooked by the weather forecasting community (Santanello et al . ). Recent studies have demonstrated the critical role of the land surface, and in particular soil moisture, on precipitation (e.g.…”

“…; Santanello et al . give comprehensive reviews). Thus, the large uncertainty of soil moisture represents an important parameter to be perturbed in CPEPS.…”

“…Its impact is put in relation to the impact of two important physical processes that need to be accurately represented in weather models to forecast convective precipitation -boundary-layer turbulence, and cloud microphysics and its interaction with aerosols (Clark et al, 2016). Since many convective-scale phenomena are forced from the lower boundary condition, surface and soil properties represent an important source of uncertainty which has been mostly overlooked by the weather forecasting community (Santanello et al, 2019). Recent studies have demonstrated the critical role of the land surface, and in particular soil moisture, on precipitation (e.g.…”

“…Soil moisture influences the partitioning of net radiative flux into latent and sensible surface heat fluxes, eventually moistening and heating the boundary layer and playing an important role in cloud development as well as in the initiation and formation of convective precipitation (Wallace and Hobbs, 2006). However, soil moisture fields are largely unknown and simple assumptions about the surface state can introduce large uncertainties (Van Weverberg et al, 2010;Santanello et al, 2019). Even nowadays, few soil moisture observations, the large subgrid variability and imperfect data assimilation techniques hamper a realistic convective-scale soil moisture analysis (Gustafsson et al, 2018;Santanello et al, 2019 give comprehensive reviews).…”